The effect of physical molecular features on complex coacervation phase separation

coacervate;polyelectrolyte;transfer matrix;Monte Carlo, ,

Lytle, Tyler

关键词: coacervate;    polyelectrolyte;    transfer matrix;    Monte Carlo, ,;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/105642/LYTLE-DISSERTATION-2019.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Complex coacervation is an associative liquid-liquid phase separation of oppositely-charged polyelectrolytes in aqueous salt solution. This phase separation is sensitive to chemical and physical molecular features making it attractive for a large number of applications. However, precisely tuning the phase behavior using specific chemistries is difficult, but physical molecular features, such as chain length, architecture, and chain polarity, can be used to precisely tune the phase behavior. To understand the link between phase separation and these molecular features, a copious amount of theoretical modeling of complex coacervates has been performed using a number of approaches such as Voorn-Overbeek theory, polymer field theory, counterion condensation and release, and liquid state theories. While these theories have given physical insights into coacervation, most of these approaches are not applicable to polymers with a high charge density, which is the relevant limit for most synthetic polyelectrolytes, and are difficult to extend to length scales associated with charge-driven self-assembly.My work has led to the development of a transfer matrix theory that captures how molecular features affect complex coacervation in the high charge density limit. This theoretical approach maps the 3-dimensional system to a 1-dimensional adsorption model, and solves this 1-dimensional adsorption model using a transfer matrix approach. Inputs to the theory are determined using Monte Carlo simulation. Qualitative matching with simulation and experimental results is achieved for bulk phase separation. In order to capture how salt valency, chain stiffness, and chain architecture influence coacervation, this theory is modified using physically-motivated arguments.Results from these modifications suggest a combinatoric entropy gain is a driving force for coacervation. This means the presence of many different polyelectrolyte chains in the coacervate phase allows for a large number of ways for them to adsorb onto each other causing an increase in entropy.Additionally, precise charge sequence effects can be captured using this transfer matrix theory. Initial simulations demonstrated that the periodicity of the charge sequence affects counterion entropy gain, which is the entropy counterions gain upon being released from a polyelectrolyte. Chains with longer runs of charged monomers increasingly localize counterions compared to chains with shorter runs of charged monomers, resulting in an increased entropy gain upon coacervation. This observation is used to modify the transfer matrix theory to capture these trends in qualitative agreement with simulation and experiment. Further investigation of more charge sequences demonstrated that charge fraction, average length and relative positions of charged monomer `runs' all influence coacervation phase behavior; this suggests precise charge sequence is a powerful method for designing coacervating systems. Since this transfer matrix theory is a useful description of coacervation phase behavior, I use it to understand interfacial properties of coacervates. This importantly sets the foundation for understanding self-assembly driven by complex coacervation. Theoretic interfacial profiles showed qualitative matching with coarse-grained molecular dynamics simulations. Interfacial tension, interfacial width, and the interfacial excess of salt are calculated showing that adding salt to the system decreases the interfacial tension allowing the interfacial width to increase with a small concentration of salt adsorbing to the interface. Addition of a neutral polymer species drastically alters the phase separation behavior and interfacial properties due to the excluded volume interaction of this species. Morphological phase diagrams for coacervate-forming block copolymers are determined using a single chain in a mean field simulation, which show structures seen in neutral block copolymer systems. Large concentrations of salt and polymer can induce the formation of coexisting morphologies. Additionally, transitions from disorder to order upon addition of salt are observed. The theoretical approach I have developed is broadly able to capture how physical molecular features affect coacervation phase behavior in the high charge density limit, and can be used to understand coacervate-driven self assembly. This theory is also capable of capturing the effect of monomer-level charge sequence effects, and a number of other ways to tune the phase behavior. This work provides a foundation to start considering more complex sequence-defined coacervate systems, such as mixtures of sequence-defined polyelectrolytes and systems which undergo hierarchal self-assembly.